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Cooking and processing food is something that is widely accepted, but many of us don't realise there may be nutritional drawbacks. High heat, as well as many of the cooking processes, alters the physical and chemical structure of food, changing how it is digested and the nutrients that are available to be absorbed. Enzymes are destroyed and nutrients can be lost from almost all forms of cooking, depending upon the type of cooking, temperature, pH, oxygen content and type of food. In addition, the processing of foods may add toxins, which can build up in the body causing a negative effect on health.

Vegetables provide a good example of how cooking reduces the quantity of valuable nutrients. With vegetables, cooking by means of boiling or steaming, produces three composition-changing actions: shrinkage due to the extrusion of vegetable juices; leaching by either boiling water or condensed steam; and hydration.

Leaching produces a higher loss of water-soluble nutrients in vegetables, which increases the longer the vegetable is boiled and the amount of water used. Between 70 and 80% of vitamins and minerals including vitamin C and B group (such as thiamine and folate), and potassium are lost during the boiling process. Steaming vegetables does not result in the loss of anywhere near as many nutrients into the water as with boiling, but steamed vegetables still may lose up to 30% of their water and water-soluble nutrients. As the nutrients leach into the cooking liquid, the full value of the vegetable can only be maintained if this is consumed as well as the solid food.

In experiments, boiling spinach and broccoli removed between 51% and 56% of folate. Thiamine (B1) has one of the highest losses by cooking - up to 80% - with complete loss from oven-roasted food, while boiled legumes had 50% thiamine loss. Riboflavin (B2) losses tend to be less but still significant; roast chicken retained only 22% of its riboflavin after cooking.

The fat-soluble vitamins, A, D, E and K, are not lost through boiling because they are not water-soluble, though they are still prone to oxidation damage through high heat. Other cooking processes though can certainly have a big impact on the levels of the fat-soluble vitamins. Oven roasting lamb chops decreased the vitamin A content by 58%. Similarly, baking fish reduced vitamin A content by 37%. Many foods lose between 19 and 57% of the carotenoids, beta-carotene and xanthophylls when cooked.

The minerals that form the most soluble salts, potassium and sodium, have the highest loss using wet (boiling and steaming) cooking methods. In an experiment, boiled fish lost 25% of its potassium and 60% of its sodium. Pressure cooking resulted in losses of 51% of sodium and 71% of potassium in mangrove seeds. Boiling of some vegetables (mushrooms and asparagus) reduces their selenium content by between 29% and 44%.

Protein loss through cooking is minimal, appearing to be around 13%. However, after cooking, proteins become harder to digest as they form crosslinks with reducing sugars, meaning the body has to work harder to break them down, which may lead to the fermentation of raw meat in the digestive system. Raw meat, such as steak tartare and sushi, are much easier to digest, although they are an acquired taste.

Cooking increases acidosis

Cooking food alters its natural state, and therefore the way it interacts with the body. Ideally, the body should have a neutral pH level, around 7.2 to 7.4, but heavily processed foods and cooked animal proteins can increase the acidity of the body. As a result, acidosis (increased acid in the body) decreases the body's ability to absorb minerals and other nutrients, energy production in the cells and the body's ability to repair damaged cells.

Heat-produced toxic compounds

The higher the cooking temperature, the more food is altered, sometimes into toxic compounds. One study found close to 3,800 heat-formed chemicals in cooked food, including a number of carcinogens. Toxins developed from cooked and processed foods include acrylamide, heterocyclic amines, nitrosamines and polycyclic aromatic hydrocarbons (PAHs). These substances are all carcinogens. Carcinogens such as acrylamide, which is found in cooked and processed foods, are not found in raw foods. PAHs, the burnt bits of food (for example, from barbecuing meat or toast), are known to be carcinogenic and are the oldest known chemical carcinogens.

In food, more than 10 out of 20 PAHs identified have been shown to be carcinogenic in experimental animals. Oral administration at various concentrations in rodents has resulted in stomach, ovarian, lymphoid, mammary and hepatic tumours. The major sources for PAH in food are during heat processing (grilling, smoking, toasting). The formation of PAH is minimal below 400oC, but the amounts increase linearly in the range 400-1000oC or when foods are in direct contact with flames.

Smoking also forms a variety of other toxic compounds including nitrogen oxides, which can form nitrites able to react with amines and amides, yielding N-nitroso compounds (Nitrosamines and Nitrosamides). Formation of these compounds occurs optimally around pH 3.5 and is catalysed by high temperatures. A significant portion of nitrosamines produced during the frying of food is also found in condensed vapours and may be breathed in by people doing the cooking. These nitroso compounds are well known cancer-causing chemicals in both human and animal studies.

Various mutagenicity (pre cancer cells) tests also showed positive results related to amino acid pyrolysis (protein cooking) products and are considered potent liver carcinogens in rats and mice. Frying at 143oC for 20 minutes only produced low activity, whereas 191oC and 210oC for up to 10 minutes gave much higher mutagen activities.

In meat, it appears that only low levels of TA98 mutagenic activity are produced when ground beef is microwaved, stewed, simmered, boiled or deep fat fried. Baking, roasting and broiling (grilling) produce moderate activity, while frying produces the highest activity.

Increases in time and temperature of cooking have been shown to increase advanced glycation end products (AGEs), which are shown to increase inflammation, particularly in diabetics. AGE products do considerable damage in the circulatory system and are increasingly associated with a range of adverse health effects from which diabetics suffer, including those leading to gangrene and blindness.

Plastics in heating, cooking and microwaves

Unfortunately, many people do not realise that plastics leach out toxic substances into the food and the rate of this release depends on heating and the type of food. So putting hot food into plastic containers or heating food in plastic containers, which is common with microwave cooking, increases the release of different toxic plastic compounds such as PVC, BPA (Bisphenol A) and plasticisers like phthalates (DEHA).

DEHA, a phthalate-like chemical added to plastics to make them more pliable, is a known endocrine (hormone) disruptor and causes testicular and reproductive defects in rats. In studies conducted on rats, DEHA has been known to cause androgenic effects. It was also found to have caused developmental toxicity in rat foetuses.

BPA has been found to cause oestrogenic effects in rats in low exposure, a change in maternal instinct in rats at one fifth the level considered safe and aneuploidy (extra or missing chromosomes) in mice. Aneuploidy is the cause of spontaneous miscarriage in humans and causes between 10 and 20 percent of birth defects, including Down's syndrome. This implicates BPA in a suite of health problems.

"Microwave safe" is not a health claim and has no bearing on the movement of chemicals into the food. Instead it is a warning that the plastic will not physically deteriorate if exposed to microwaves. That is it will not melt. Many of the microwave safe products use polyethylene instead of other plastics such as PVC as polyethylene has no plasticisers. PVC is a known liver cancer-causing agent.

Teflon cookware common in most kitchens is a high temperature cookware made with material that enables it to heat consistently at a lower temperature to prevent burning. It is more resistant to damage caused by sudden temperature changes and maintains a non-stick surface. Teflon cookware is formulated from Polytetrafluoroethylene (PTFE) and Perfluorooctanoic acid (POFA), both toxic substances which are released into the food and into the air.

Cooking with Teflon enables these gases to be released and penetrate the alveoli creating respiratory problems through direct damage to cell membranes of the lungs, elevating cholinesterase activity and increasing levels of inorganic fluorides in the human body.

Although research on the effects of PTFE on human health is limited, extensive animal studies have shown inhalation exposure produces adverse health effects in several organs and at higher concentrations even death. Rats exposed to high concentrations developed haemorrhages, oedema, fibrin deposition in lungs and damage to the proximal tubule of the kidney. Most alarming, however, PTFE releases toxic pyrolysis products in the air that can cause rapid death of birds. Exposure of budgerigars to PTFE pyrolysis products in the air for only nine minutes produced severe clinical signs, lesions and death of 31 of 32 birds. A similar effect on five cockatiels was also observed in an incident where all five had died within 30 minutes of exposure to an over-heated frying pan containing PTFE.

Of additional concern in the manufacture of Teflon cookware is the inclusion of Perfluorooctanoic acid (PFOA) in its non-stick coating. Perfluorooctanoic acid is linked to damaging the human immune system, altering the endocrine system, causing infertility, damaging children's health and producing development problems, and has been found to be carcinogenic. One study found PFOA production workers in the US had a three-fold increase in developing prostate cancer. The US EPA state that not only is PFOA a likely carcinogen, exposure to the chemical also impairs the fertility of women. Exposure to PFOA in utero (to the foetus) has been linked with reductions in newborn birth weight and adverse effects on the skeletal and organ development of the baby. Exposure to POFA from Teflon cookware alters the endocrine system including decreased levels of reproductive hormones and disrupted thyroid hormone regulation.

Unhealthy indoor air

Other than the emissions from Teflon cookware into the air, a lesser known side effect of cooking is the creation of indoor combustion air pollutants. Carbon monoxide (CO), nitrogen dioxide (NO2), and particulate matter (PM) are harmful air pollutants that pose significant short and long-term health risks. These same pollutants are also some of the most common contributors to unhealthy air inside homes, due in part to cooking. Researchers now understand that the process of cooking food and even simply operating stoves - particularly gas appliances - can emit a cocktail of potentially hazardous chemicals and compounds. Within our homes, these pollutants are less diluted than they are outdoors, and in the absence of proper ventilation, they often are trapped inside. Literally millions of people are routinely being exposed to air pollutants at levels that we don't allow outdoors.

Cooking and weight gain

Cooking is a universal human behaviour that has been proposed to function partly as a mechanism for increasing dietary net energy gain. Research shows that cooked foods are a major contributing factor to the epidemic of weight gain and the obesity crisis we see now. A long time ago, farmers found out that animals eating raw food put on a lot of lean mass but not much fat or the weight needed to make good profits. So farmers started processing the food to get bigger weight gains. In experiments mice fed processed grains put on significantly more weight and became obese compared to mice fed whole grains in their natural unprocessed form.

An explanation for this is that processing and cooking increases the energy gained from carbohydrate, protein and fat sources. In fats the cellular structure of many foods constrains their digestibility. For example, oilseeds have cell walls composed mainly of indigestible non-starch polysaccharides and store their lipids in oil bodies, intracellular, spherical organelles coated by oleosin proteins. These features hinder digestive lipases from accessing the encapsulated lipids, which may explain why unprocessed (raw/whole; RW) nuts and other oilseeds have high measured lipid and energy content, but display lower digestibility. Cooking and/or mechanical processing tears cell walls and disrupts oil bodies, promoting lipid release. This shows that processing could increase lipid digestibility, as, unguarded by cell walls and oleosins, the freed lipids are likely more accessible to lipases.

Prebiotics, through the metabolism of the gut microbiome, have also been linked to satiety effects and foods that contain fibre, protein, and plant-based fat tend to be the most filling. These nutrients slow down digestion and the absorption of nutrients, a process that helps you feel physically full for longer, and also means lower blood sugar and insulin spikes. While all unprocessed plant sources are rich in prebiotics, leeks are rich in fructan and cellulose fibres (types of prebiotics) are long enough to survive all the way down the GI tract. However, cooking shortens the fibre chain, so this vegetable should be eaten raw or lightly cooked.

DISCLAIMER: Dr Peter Dingle is a researcher, educator and public health advocate. He has a PhD in the field of environmental toxicology and is not a medical doctor.

Dr Peter Dingle (PhD) has spent the past 30 years as a researcher, educator, author and advocate for a common sense approach to health and wellbeing. He has a PhD in the field of environmental toxicology and is not a medical doctor. He is Australia’s leading motivational health speaker and has 14 books in publication.